Du kan bruge union all for at fjerne pivot dit datasæt og derefter aggregering:
select disease, count(*) total
from (
select disease from mytable
union all select additional_disease1 from mytable
union all select additional_disease2 from mytable
union all select additional_disease3 from mytable
union all select additional_disease4 from mytable
) t
group by disease
order by total desc, disease